Stoichiometry

The stoichiometry (from gr στοιχειον stoicheion " raw material " and μέτρον metron " measure " ) is a fundamental mathematical tool in chemistry. With their help, a reaction will be charged the actual proportions ( equation ) and quantities from the qualitative knowledge of the reactants and products. In the chemical stoichiometry colloquial language does not refer to the (usually trivial ) calculation, but their result. The implementation of a reaction is often " unstöchiometrisch " meaning that at least one reactant is used in excess and, therefore, remains in part.

Basics

The calculation of the foundations of modern stoichiometry based (also historically ) on the following laws:

The laws of stoichiometry are derived so forth from the knowledge of the structure of matter made ​​up of atoms and molecules.

Terms

Stoichiometric balance

In the stoichiometric calculations it is about to calculate the amount of starting material, the reactant, which must be used in a chemical reaction. The calculation can be reversed, so that you can, knowing the amount of reactant ( s) determine the amount of product ( s).

To any reaction accounted for, is proceeding to a more general mnemonic. For a simple chemical reaction, it is for example:

  • With the stoichiometric ratios (also called stoichiometric coefficients ); in the German standard DIN 32642 " Symbolic description of chemical reactions " is this the term " stoichiometric number " is recommended.

As can be set up for a different reaction equations

Or,

Stoichiometric ratios must be set before the financial statements. The following applies:

  • Reactants always get a negative stoichiometric ratio
  • Products have a positive stoichiometric ratio
  • And accompanying substances ( substances that do not participate in the reaction ) get a stoichiometric ratio of 0

In the reaction, the proportions change (more precisely, the amount of substance ( n )) of the reactants in the extent that the stoichiometric ratios seem to suggest. The stoichiometric balance for the reactant i and k is calculated as:

Through simple transformation, one obtains for the set operation

And correspondingly for the operation flow

Turnover ( Xi)

This is a term used in chemical reaction engineering and describes how much reactant reacted at a reaction. With the turnover ( grad) that specifies which portion of a starting material leaving the reactor was converted by chemical reaction into other chemical substances. In mathematical terms: sales, the percentage of reacted amount of a component ni i with respect to the amount, 0

  • Where ni is the remaining amount of component i is

If several starting materials involved, the degree of conversion is specified by convention for those material which is limiting or present in deficiency.

Yield

The yield is a term used in chemical reaction engineering. The yield is the amount of product P, based on the main component (K). The main component is, by convention, one substance is present in an amount less than that corresponding to the stoichiometry of the reaction.

Selectivity

Selectivity is a term used in chemical reaction engineering. The selectivity of a chemical reaction or a reactor indicates what proportion of the total unreacted reactant was implemented taking into account the stoichiometry in the desired target product. In general, not all the molecules set to the desired product to other products may be produced by secondary or competing reactions.

Conversion, yield and selectivity

Combining the definitions of conversion, yield and selectivity of each other, one obtains a simple relation of the three sizes:

This means that if there is only one possible response, S = 1 and the yield Y is the turnover X.

Mathematics Product (chemistry) Law of definite proportions Digital Object Identifier
750319
de